A militiaman stands guard in Mazar-e-Sharif north of Kabul, Afghanistan. Rahmat Gul/AP

US Defence Secretary Lloyd Austin called on Friday for international pressure to force a deal between the Kabul government and the Taliban hours after President Joe Biden issued a staunch defence of the US withdrawal on Thursday.

Austin said in a tweet that "the security situation in Afghanistan only argues more for international pressure to have a negotiated political settlement to end this conflict, and give the Afghan people (the) government they want and they deserve."

"The entire world can help by continuing this push."

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Afghan security forces vow to retake districts lost to Taliban

Taliban capture key Kandahar district from fleeing Afghan troops

On the other hand Taliban claimed to be in control of 85 per cent of Afghanistan as its fighters had seized two crossings in western Afghanistan — completing an arc of territory from the Iranian border to the frontier with China.

US Defence Secretary Lloyd Austin speaks during a press conference. File photo

Taliban have also seized key border crossings with Iran and Turkmenistan, part of a sweeping offensive launched as US troops pull out of the war-torn nation.

A delegation of Taliban officials said in Moscow that they controlled about 250 of Afghanistan's nearly 400 districts — a claim impossible to independently verify, and disputed by the government.

Their latest offensive started in early May as the United States and NATO began the final troop withdrawal following a 20-year occupation sparked by the September 11 attacks.

Mohammad Naim (left), Mawlawi Shahabuddin Dilawar and Suhil Shaheen, from the Afghan Taliban's movement, attend a news conference in Moscow, Russia, on Friday. AP

The Taliban and government were supposed to have agreed a political roadmap for the country at peace talks in Doha, but they have largely fizzled out after months of deadlock, and the insurgents appear now to have their sights set on a military victory.

The Taliban advances have raised fears Afghan security forces may swiftly become demoralised without the vital air support provided by the Americans, and collapse.

Mehrullah, who like many Afghans uses only one name, was part of a battalion of around 1,000 soldiers defending the border crossing with Tajikistan near Kunduz city, in the far north.

"We were under siege in Shir Khan Bandar for a week. Our supply route was cut off," said the 27-year-old, who spoke of confusion and a lack of communication between units on the ground.

Militiamen stand guard at their office in Mazar-e-Sharif north of Kabul, Afghanistan. AP

Taliban spokesman Zabihullah Mujahid separately told the media reporters that their fighters had captured the border town of Islam Qala on the Iranian frontier and the Torghundi crossing with Turkmenistan.

Herat governor spokesman Jilani Farhad said the authorities were preparing to deploy new troops to retake Islam Qala port, the biggest trade crossing between Iran and Afghanistan.

The Afghan government has repeatedly dismissed the Taliban's gains as having little strategic value, but the seizure of multiple border crossings along with mineral-rich areas will likely fill the group's coffers with several sources of new revenue.